SEI»ECTION OF COMMUNICATIONS OPERATORS 



235 



ing noise. Tests on random samples from 

 the military population have shown that, 

 under difficult conditions, the best listeners 

 can consistently understand twice as many 

 messages as bad listeners (9, pp. 12-14; 5, 

 pp. 3-8). The results of a typical test are 

 shown in Fig. 2. It is obvious that the 

 random assignment to telephone posts of any 

 personnel, whether well or poorly endowed 

 with this ability, is a hazard to communica- 

 tion. 



The ability to listen in noise shows little 

 correlation with general intelligence (9, p. 

 25). Experiments have also shown that this 

 talent cannot be measured by conventional 

 medical tests for acuity of hearing, because 

 (1) against a noisy background a signal can. 



SCORES FOR 17 

 ENLISTED tlSTENERS 



H5 DB t K LL 



F J MM IM TO 



Fig. 2. How listeners differ in their ability to 

 hear words in the presence of noise 



not be heard unless it exceeds the hearing 

 threshold of all but the most seriously deaf- 

 ened individuals, and because (2) the abil- 

 ity to understand words is a more complex 

 function than that measured by the simple 

 and non-verbal sounds commonly employed 

 in tests for aural acuity (5, pp. 14-15). 



Three different tests (Auditory Tests Nos. 

 2, 4, and 8), prepared by the Psycho- Acous- 

 tic Laboratory, Harvard University, are 

 available which test this specific ability (9, 

 pp. 1-2, and Appendix A). Each consists 

 of speech materials (either single words or 

 sentences) recorded on phonograph discs 

 against a background of masking noise; each 

 may be administered simultaneously, over 

 playback, amplifier, and earphones, to 



groups of 50 or more men within one-half 

 hour; and each has a reliability of .90 or 

 better. One of these tests (Auditory Test 

 No. 8) is of the multiple-choice type, and 

 may be scored on an IBM machine. Avail- 

 able evidence indicates that any one of these 

 tests will yield a valid index to an individ- 

 ual's general ability to understand messages 

 over a communication circuit under condi- 

 tions made difficult by interfering noise. 



Tests for Talkers, Human beings differ 

 even more markedly in their capacity to 

 speak so as to be understood in a noisy en- 

 vironment, than in their capacity to under- 



Fig. 3. How greatly a group of talkers can differ 

 in their ability to make words understood by a 

 group of listeners in the presence of noise 



stand what has been said to them. In one 

 group of untrained subjects, for example, 

 speaking under identical conditions, the best 

 talkers were able to make heard five times 

 as many messages as the worst (10, IC-60, 

 pp. 6-8; 5, pp. 8-9). Another group gave 

 the results shown in Fig. 3. 



Extended experimental analysis indicates 

 that the following attributes of a voice 

 largely determine its audibility in noise 

 (10, IC-81, pp. 1-3, 9-11, 27-29; and 20): 



1 . The loudness a talker is able to sustain 

 when instructed to speak in the "loudest 

 voice possible without strain." 



2. "Consonant strength" — the amount of 



